Whether she’s preparing for a mission aboard the International Space Station or traveling for a vacation, Sunita Williams always packs one thing: running shoes.

“Everywhere I go I always have a pair of running shoes with me,” says Williams, a 52-year-old NASA astronaut. “That’s just part of the standard package.”

Her parents encouraged her to lead an active lifestyle while she grew up in Needham, Massachusetts, a suburb of Boston. She was a long-distance swimmer who “grew up doing the 5 a.m. swimming thing.”

It wasn’t until her senior year of high school that she started running track in the spring—and even then, it was more to prepare for college at the Naval Academy, which required cadets to run.

As a 17-year-old, Williams ran the 1983 Boston Marathon as a bandit. She didn’t have a pair of running shoes, so she wore basketball sneakers.

“I shortly figured out that wasn’t a good idea,” says Williams, who has now run Boston five times.

That spring, coaches noticed her speed and asked her to join the cross-country team.

“I was like, ‘Yeah, sure,’” says Williams. “I’m not a super-duper athlete. I’m a good long distance guy, but the Naval Academy is 10 percent women, so there’s slim pickings.”

She ran three years for the Midshipmen, ditched the basketball sneakers for running shoes, and started carrying running shoes wherever she went. As a Navy Officer she ran on the ship’s deck and when it docked. She's run in Norway, the Mediterranean, Spain, France, Italy, Israel, Greece, and the United Arab Emirates.

“All those places we pulled in the ship, I went for a run,” says Williams.

In 1998, Williams became part of the 17th group of NASA astronauts, which gave her the chance to carry her shoes to Japan, Canada, Russia, and India.

It also gave her the chance to run in space.

In 2007, Williams signed up to run the Boston Marathon with her sister, but a flight delay kept her up on the International Space Station. So she decided to run it anyway—at night on the station’s treadmill.

She lifted the harness over her shoulders and around her waist and attached it to two bungees, which pull the astronaut to the treadmill and counter against the microgravity that makes it possible for astronauts to float.

While running, she watched video highlights of the 2006 Boston Marathon—courtesy of race officials—from a care package from her parents. Bags of water and lemonade, velcroed to the walls near the treadmill, kept her hydrated.

For the last hour, fellow astronauts threw slices of oranges, which she grabbed while they floated by. Williams finished in 4:23:10, completely soaked in sweat.

“One funny thing about running in space is you sweat a lot,” Williams says. “On Earth, you sweat, but if it’s windy or cold, it dries up off of you.”

“In space, it sort of globs on you. It’s big balls of water sticking on you. I was absolutely soaking wet, and the only way I could get dry was to take my clothes off and change. I was thinking about doing that during the marathon, but I said, ‘Forget it. I’m just going to keep going.”

Five years later, Williams met CNN’s Dr. Sanjay Gupta, a longtime promoter of the importance of exercise in a person’s daily life. Dr. Gupta invited Williams to race the Malibu Triathlon. She declined, telling him that she would be in space.

“He said, ‘Oh, could you do it in space?’” says Williams, who presented the idea to NASA trainers.

The trainers loved the idea. They designed a special workout on a weightlifting device called “ARED” to use the same muscles in swimming. She biked on a stationary cycle with no seat, and ran on the treadmill.

“We talked about how long the swim was, and for me it would take about 20-25 minutes,” says Williams, who finished the triathlon in 1:48:33. “It was a top-to-bottom, head-to-toe, lifting continuously.”

The record of being the first person to race a triathlon in space wasn’t William’s motivation. It was her belief that exercise is important for everyone.

“Even astronauts have to get up and work out,” Williams says. “It’s not like you can just float around up there.”

On Earth, Williams estimates that she runs at least five miles, five days a week, with a 10-mile run on the weekends.

Sometimes, it’s difficult to find time, but running is a mental relaxation for her.

Seeing the world from space and traveling to different countries has changed Williams’ view of what running means to her.

While working in Russia, Williams was surprised at how many runners there were.

“I think running is one of those things where you grab your shoes and you can go anywhere,” Williams says. “I think people feel pretty free to be able to do that.”

“I hope to carry my shoes to more cool places in the world where I haven’t in the past and go running and see that people have that choice. That they can go running anywhere, at any time.”